Wear mechanisms in palmyra-fruit-fiber-reinforced polymer composites

14 October 2016
Sankar Irullappasamy, Durairaj Ravindran, and Irulappasamy Siva
The effect of several different fiber-surface chemical treatments is investigated for palmyra fruit fiber/polyester samples.

There are many challenges involved in using natural fibers to reinforce polymers. In fiber-reinforced polymer structures, the overall material strength generally depends on a few primary factors (e.g., fiber compatibility, loading, and orientation). Since these primary factors have been optimized for polymer reinforcement, it is now necessary to investigate secondary factors (including fiber–matrix adhesion, special additives, and the method of fabrication) that also affect the material strength. Among these secondary factors, fiber–matrix adhesion plays a vital role in determining composite behavior and it is therefore important to identify the major influences on this parameter.

To date, many approaches for enhancing the interfacial adhesion in natural-fiber-reinforced polymer composites have been reported. Fiber-surface modification was the main focus in most of these studies, but a variety of problems have been encountered. For instance, low mechanical strengths—caused by reduced bonding at the fiber–matrix interface—have been reported by several groups.1–3 In addition, tribological issues (e.g., low wear resistance) have been noted.4–8

To overcome these mechanical strength and tribological issues, we have investigated the use of a fiber-surface chemical treatment to increase the interfacial bonding in fiber-reinforced (with fibers from the palmyra fruit) polyester composites.8 Indeed, we have studied the unique fiber-surface-modification effects of various chemical agents. Our study builds upon extensive work in which the influence of primary fabrication parameters (e.g., fiber loading and orientation) for palmyra-fruit-fiber-reinforced polyester composites were reported.6 In particular, we used an alkali treatment—sodium hydroxide (NaOH)—which has previously been found to physically improve the gripping ability of natural-fiber surfaces and to activate the inactive hydroxyl groups on the fiber surface to provide better wettability of the polymer matrix during composite fabrication.3 In addition, we use a silane treatment (with trichlorovinyl silane) to ensure the development of interfacial adhesion. This treatment causes the formation of an interpenetrating polymer network between the fiber surfaces and the polymer matrix.4, 5 In addition to the NaOH treatment, we have also introduced, and analyzed the effects of, a novel calcium hydroxide—Ca(OH)2—alkali treatment.

The effects of our fiber-surface chemical treatments on the static mechanical properties of palmyra-fruit-fiber-reinforced polyester composites are illustrated by the results given in Table 1. We find that the mechanical strength properties of the NaOH-treated sample were superior to both the untreated sample and the Ca(OH)2-treated and silane-treated samples. In addition to the structural characteristics of our composites, we used the pin-on-disc wear testing method to analyze the tribological behavior of the samples. The specific wear rates and friction coefficients that we thus measured for our samples are illustrated in Figure 1.

Mechanical properties of the palmyra-fruit-fiber-reinforced polyester composite samples subjected to a variety of surface chemical treatments.

CompositeTensile strengthFlexural strengthImpact strength
sample(MPa)(MPa)(KJ/m2)
UTC4049256.93
ATC5654318.72
CTC4850260.14
STC5051272.39

Effect of fiber-surface chemical treatment on the palmyra-fruit-fiber-reinforced polyester composites in terms of the (a) specific wear rate and (b) coefficient of friction, for three different sliding velocities. UTC: Untreated composite. ATC: Sodium hydroxide-treated composite. CTC: Calcium hydroxide-treated composite. STC: Silane-treated composite.

We used scanning electron microscopy (SEM) to explore the wear failure mechanism of our samples (see Figure 2). The major wear-failure mechanisms that we identified in the untreated sample—see Figure 2(a)—were fiber debonding and fiber cracking. We recognize fiber debonding by its associated deposits of wax, impurities, and other decomposable substances, all of which reduce the level of bonding between the fiber and the polymer matrix, and thus lead to debonding under load conditions.8–11 Microlevel fractures along the axial and radial directions of the fibers are characteristic of fiber cracking.8–10 We find, however, that these major wear-failure mechanisms can mostly be prevented with the use of our surface-treated palmyra fruit fibers: see Figure 2(b–d). Other important wear mechanisms that are revealed by our SEM images include surface fatigue (removal of material on the sliding surface caused by fluctuating stresses8, 9,12), plastic deformation (micropeaks in the wavy surface, i.e., around pits that have undergone plastic flow caused by the pressure that develops because of the asperity of the counter body8–10), and crazing (localized formation of fine cracks, under load conditions: crazes are bridged by fibrils and they cause the initiation/propagation of cracks in brittle polymers8, 9,13).


Scanning electron microscope images of the (a) UTC, (b) ATC, (c) CTC, and (d) STC polymer composite samples. Labels indicate the identified wear mechanisms, i.e., fiber debonding (FD),8–11 crazing (C),8,9,13 resin cracks (RC),8,9 fiber cracks (FC),8–10 surface fatigue (SF),8,9,12 debris (D),8,10,14,15 grooving (G),8,9,11 and plastic deformation (PD).8–10

In general, during the wear of natural-fiber-reinforced polymer composites, the counter body erodes the sliding body and matter (debris) detaches from the sliding surface in the form of large patches of fiber and plastic (these patches are also instantly removed from the sliding interface). Material systems in which there is a large removal of this debris during sliding are known as low-wear-resistance materials. In other words, in low-wear-resistance materials, the rubbing force is sufficient to detach the matter (as large patches) from the sliding surface because of weak interfacial bonding between the fiber and the matrix. Our results indicate that once we increase the fiber–matrix interfacial bonding strength (i.e., by treating the palmyra fruit fibers with NaOH before composite fabrication), we enable the materials to resist such detachments. Rather than large patches, the material is lost as tiny debris particles during wear (and the materials can be said to have a high wear resistance). In addition, the wear debris may be retained between the sliding and rubbing surfaces for a short time.

In summary, we have investigated the effects of using a variety of chemical treatments to improve the mechanical and tribological characteristics of palmyra fruit fibers when they are used to reinforce polymer composites. We find that a NaOH treatment, compared with other surface treatments—i.e., with Ca(OH)2 or silane—provides samples that exhibit better mechanical strength and wear behavior. It is also thought that blending nanomaterials with the polymer matrix during fabrication may increase the load transfer between the fiber and matrix phases, and may thus further improve the structural characteristics of the resulting composites. In our future work we therefore plan to continue our palmyra fruit fiber/polyester composite research by considering the addition of nanomaterial to the matrix phase.


Authors

Sankar Irullappasamy
National Engineering College

Durairaj Ravindran
National Engineering College

Sankar Irullappasamy received his undergraduate degree in mechanical engineering and postgraduate degree in manufacturing engineering from Anna University, India. He is currently working as an assistant professor in the Department of Mechanical Engineering. His scientific interests include polyester composite materials, natural fibers, and wear failure analysis.

Irulappasamy Siva
Kalasalingam University

I. Siva is an associate professor.


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DOI:  10.2417/spepro.006745